1. Field of the Invention
The present invention relates primarily to digital printing apparatus and methods, and more particularly to imaging of lithographic printing-plate constructions on- or off-press using digitally controlled laser output.
2. Description of the Related Art
In offset lithography, a printable image is present on a printing member as a pattern of ink-accepting (oleophilic) and ink-rejecting (oleophobic) surface areas. Once applied to these areas, ink can be efficiently transferred to a recording medium in the imagewise pattern with substantial fidelity. Dry printing systems utilize printing members whose ink-repellent portions are sufficiently phobic to ink as to permit its direct application. Ink applied uniformly to the printing member is transferred to the recording medium only in the imagewise pattern. Typically, the printing member first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to inking. The fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
An alternative to traditional wet printing is single-fluid ink systems, which are emulsions of an oleophilic ink phase and an aqueous or nonaqueous polar phase. The ink is applied directly to a wet plate without prior application of dampening fluid. The polar phase wets non-image, hydrophilic portions of the plate surface, forming a weak boundary layer that prevents adsorption of the oleophilic ink component. The ink component does, however, adsorb onto the oleophilic image portions of the plate. Typically, single-fluid inks are "water-in-oil" emulsions containing up to 80% of a hydrophilic liquid such as water or a polyhydric alcohol (e.g., ethylene glycol).
Photographic platemaking processes tend to be time-consuming and require facilities and equipment adequate to support the necessary chemistry. To circumvent these shortcomings, practitioners have developed a number of electronic alternatives to plate imaging. With these systems, digitally controlled devices alter the ink-receptivity of blank plates in a pattern representative of the image to be printed. U.S. Pat. Nos. 5,339,737 and 5,783,364, the entire disclosures of which are hereby incorporated by reference, disclose a variety of lithographic plate configurations for use with imaging apparatus that operate by laser discharge. These include wet plates as described above and dry plates to which ink is applied directly. These plates may be imaged on a stand-alone platemaker or directly on-press.
In the former case, although the most cumbersome aspects of traditional platemaking are avoided, plates must be manually (and sequentially) loaded onto the platemaker, imaged, inspected, then transferred to the press and mounted to their respective plate cylinders. This involves a substantial amount of handling that can damage the plate, which is vulnerable--both before and after it is imaged--to damage from abrasion. Indeed, even fingerprints can interfere with plate performance by altering the affinity characteristics of the affected areas.
The ability to image on-press obviously reduces the possibility of handling damage substantially, but does not eliminate it. Plates must still be removed from their packaging and mounted to the press; in the case of ablation-type plates, it is frequently necessary to clean the plates to remove imaging debris, an operation that can result in abrasion if performed improperly. Indeed, lithographic printing plates can suffer damage even without handling: airborne debris, environmental contamination, movement of the packaged plates and the mere passage of time can inflict various stresses that interfere with ultimate plate performance.
To protect the plate during packaging, shipment and use, manufacturers may add a peelable barrier sheet to the final construction. As discussed, for example, in the '737 patent, this layer adheres to the surface of the plate, protecting it against damage and environmental exposure, and may be removed following imaging. But this sheet can itself damage the plate if the degree of adhesion is inappropriate or if carelessly removed, and in any case adds cost to the plate and its removal imposes an additional processing step.
U.S. Pat. No. 5,807,658 discloses wet lithographic printing plates that are provided with a protective layer serving a variety of beneficial functions, and which, desirably, washes away during the printing make-ready process. The protective layers disclosed in this patent, however, are applied by conventional coating techniques operating at atmospheric pressure. They are not amenable to application, for example, using the vacuum techniques by which the other plate layers are applied, and consequently the plates cannot be manufactured in a single pass.